Mitsuki graduated from the same Galaxy Police Academy class Kiyone graduated from, but at a lower score. She is a very ambitious person, and will stoop to anything to get ahead of Kiyone... even capturing her when Kiyone got implicated in Ayeka's problems with Jurai.

At the end of the series, Mitsuki was demoted to being Kiyone's errand girl, following Kiyone's promotion to Detective Sergeant and Mihoshi's promotion to Chief Criminal Prevention Officer.

(source: Wikipedia)